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Design for Test Report ​

Design for Manufacturing is a vital part of the PCBA design process, we feel that Design for Test is just as important. This tool makes it easy and straight forward to verify that you have designed a PCBA that can be easily tested using standard bed-of-nails test fixtures.

Getting Started ​

The overall process to create a DFT Report is:

  • Upload a IPC-D-356 Netlist
  • Identify required test points
  • Identify locating holes
  • Review report

Fixture Design 101 ​

To create a Design for Test Report, you will need to generate or create the following:

  • IPC-D-356 Netlist
  • Test Plan or Specification
  • Locating Methods

IPC-D-356 Netlist ​

Essentially, an IPC-D-356/IPC-356 Netlist is a simple ASCII-based file format that contains netlist information along with X/Y coordinates, reference designators, and pin numbers. Most importantly, it is supported by all design tools. This makes it perfect for easily capturing information to help create test fixtures.

When using this file, it is important to remember that it contains all accessible pads on the top and bottom of a PCB, whether these are test points, through-hole connectors, or BGA pads.

Due to this, you will need to make sure that you only select dedicated test points or through-hole connectors as your test points (don't worry, we'll help with filtering these out).

Test Plan or Specification Development ​

While it would be great to PROBE EVERYTHING, in reality, you want to only add the test points that you'll need access to in a fixture. This allows more robust probes to be used and helps improve the reliability and durability of the fixture.

The best way to identify the test points required is to develop a test plan or test specification. We have a great series on our blog, which you can find here.

Once you've developed a test plan, you will have a great idea of which test points will need to be selected when generating a DFT Report.

Test Plan Suggestions ​

Here is a simple suggestion for getting started with creating a test plan. For almost all devices, the following will need to be implemented:

  • Short Circuit Tests
    • Test all power inputs/outputs for short circuits
    • For example, this can be a resistive check (think DMM) or a power-on test with a low (~10mA) current limit
  • Power-On Voltage Tests
    • Apply power to the main input (battery/charger in, etc.)
    • Measure power input and voltage regulator output voltages
  • MCU Progamming Tests
    • Program all of the MCUs/FPGAs that are on PCBA
    • This can also include programming SPI memory or EEPROMs
  • Functional Tests
    • Create tests for each functional "block" on the device under test (DUT)
    • These will be specific to the device/product that is being developed
    • Examples
      • MCU controlled voltage tests
      • Motor/Stepper Drivers
      • LED/Display Tests
      • Battery Charger and Fuel Guage Circuits
      • WiFi/BLE Connectivity
      • Digital and Analog IO

Locating Methods ​

The simplest and most reliable way to locate a DUT is to use mounting holes and/or tooling holes. At FixturFab, we refer to these features as Guide Pins.

If no mounting or tooling holes are available on the DUT, then a customized cradle will need to be designed and fabricated.

Design for Test 101 ​

Here are some simple suggestions to help design your PCBAs with test fixtures in mind:

  • The larger the test point to test point spacing the better
    • Ideal: >= 2.54mm (100mil)
    • Recommended Minimum: >= 1.9mm (75mil)
    • Absolute Minimum: >= 1.27mm (50mil)
  • Make Test Points at least 0.8mm in diameter (1mm) preferred
  • Place all Test Points on the same side
    • If you need to probe through-hole components, place these on the opposite side (soldered side) of the through-hole components
    • If needed, make your test points spacing and/or pad size smaller before resorting to double-sided probing
    • Really, while this can be done, it greatly increases the cost and complexity of your test fixture.
  • Add mounting holes or tooling holes
    • At least 2 1mm > holes
  • Keep Test Points 1.27mm (50mil) away from components